This invention relates generally to the manufacture of printed circuit boards, and more specifically, to manufacturing defect analyzers.
Printed circuit boards are typically tested during the manufacturing process to determine whether the boards contain manufacturing defects. In this way, defective printed circuit boards are identified before being incorporated into electronic products, thereby minimizing the chance that the electronic products will fail prematurely in the field.
One method of testing printed circuit boards is known as card edge testing, which includes attaching a test device to input and output ports of a board under test (BUT), applying test signals to selected input ports, detecting response signals at selected output ports, and comparing the response signals to expected results. The applied test signals typically instruct the BUT to perform a series of predetermined functions. The detected response signals are then analyzed to determine whether the BUT performed the predetermined functions correctly.
However, card edge testing has certain limitations. In general, card edge testing is most useful for testing printed circuit boards that have a relatively small number of components, which are capable of performing only a limited number of functions. However, as the number of components mounted to printed circuit boards has increased, the number of functions that the boards perform has also increased. As a result, card edge testing no longer provides the fault coverage that many printed circuit board manufacturers require.
An improvement to card edge testing is in-circuit testing, which includes attaching a fixture, called a "bed-of-nails," to certain nodes on the BUT, applying test signals to selected nodes, and evaluating response signals detected at other selected nodes. The bed-of-nails fixture typically isolates a small number of components from the remaining components on the BUT. Functional testing is then performed on the isolated components. In-circuit testing is typically repeated until all of the components on the BUT have been tested. Board manufacturers have used in-circuit testing to test relatively large printed circuit boards with many components.
Nevertheless, in-circuit testing also has certain limitations. For example, both active and passive components must be spaced far enough apart on the BUT so that nodes are accessible to the bed-of-nails fixture. However, this spacing requirement cannot always be satisfied on densely populated printed circuit boards. It is particularly difficult to access the nodes on printed circuit boards that are designed using PCMCIA technology, which is typically used in the manufacture of boards for laptop computers.
Also, in-circuit testing generally does not provide complete fault coverage. For example, in-circuit testing cannot reliably detect missing by-pass capacitors, joints with insufficient amounts of solder, bent leads on semiconductor packages, or improperly aligned components.
Another method of verifying printed circuit boards is generally known as optical inspection. Originally, optical inspection was performed by human inspectors using either the naked eye or a microscope. At certain stages of the printed circuit board manufacturing process, human inspectors typically looked for missing components, extra components, improperly oriented components, tilted semiconductor packages, crooked leads, faulty solder joints, or solder bridges between closely-spaced leads. Further, by combining visual inspection with in-circuit testing, many board manufacturers have achieved nearly 100% fault coverage. However, as printed circuit boards became more complex, visual inspection of boards proved to be slow, inaccurate and expensive.
An improvement to visual inspection is automatic optical inspection (AOI), which generally uses an illumination device and a series of video cameras mounted in a fixture. A test computer typically controls movement of the fixture relative to the BUT, thereby allowing the cameras to scan and obtain images of a surface of the BUT. The test computer then digitizes the images for subsequent analysis. Because AOI devices are used to perform non-contact testing, they are generally unaffected by inaccessible nodes on densely populated printed circuit boards. Also, over the long term, AOI devices tend to be more accurate than human inspectors.
Nevertheless, AOI devices also have some disadvantages. For example, in a typical AOI device, video cameras capture light and color characteristics of the surfaces of the BUT, the components, and the solder joints within their respective fields of view. The AOI device then converts the captured light and color information into digitized image data, which is analyzed by the test computer. However, the surfaces of the BUT, the components, and the solder joints each have a relatively wide range of acceptable parameters relating to reflectivity, color, and texture. This means that in order to obtain accurate results, AOI devices must be programmed and operated by highly trained technical personnel. Even if highly trained personnel are used, AOI sometimes results in either the rejection of good boards or the acceptance of faulty boards.
Another limitation of AOI devices is that the characteristics of both the illuminators and the video cameras can vary from AOI device to AOI device. As a result, a test program written for one AOI device might not function properly on another AOI device. Finally, AOI devices are generally slow, thereby making them less adaptable to some high speed manufacturing processes.
Accordingly, it would be desirable to have a reliable way of detecting missing components, extra components, improperly oriented components, tilted semiconductor packages, bent leads, faulty solder joints, and solder bridges between closely-spaced leads on densely populated printed circuit boards. It would also be desirable to have a manufacturing defect analyzer that locates faults quickly, is easy to program and operate, and can run test programs developed for other defect analyzers without modification.